Control of thermal energy in optical devices

ABSTRACT

The optical device includes an optical modulator positioned on a base. The modulator includes a ridge extending upward from the base. The ridge includes an electro-absorption medium through which light signals are guided. A thermal conductor is positioned so as to conduct thermal energy away from the ridge. The distance between the thermal conductor and the ridge changes along a length of at least a portion of the ridge.

FIELD

The present invention relates to optical devices, and particularly tooptical devices associated with heat generation.

BACKGROUND

Optical links transmit light signals from a transmitter to a receiver.Increasing the power of the light signals can increase the length ofthese links. Components such as modulators are often used in theselinks; however, there is often a limit to the level of optical powerthat can be handled by these components. For instance, modulatorsgenerate a photocurrent during operation. This photocurrent leads tosubstantial heating of the component. As the power level of the opticalsignals increases, the level of heating also increases. This heating candamage the component and cause other difficulties. For instance, theoperating wavelength of the modulator shifts in response to thisheating. As a result, the efficiency of the modulator drops as theheating level increases. As a result, there is a need for opticalcomponents that are suitable for use with increased optical powerlevels.

SUMMARY

An optical device includes an optical modulator positioned on a base.The modulator includes a ridge extending upward from the base. The ridgeincludes an electro-absorption medium through which light signals areguided. A thermal conductor is positioned so as to conduct thermalenergy away from the ridge. The distance between the thermal conductorand the ridge changes along at least a portion of the ridge. In someinstances, the thermal conductor is one of multiple thermal conductorsand the ridge is positioned between thermal conductors.

In some instances, the modulator has an entry side through which lightsignals enter the modulator and an exit side through which light signalsexit the modulator. The distance between the thermal conductor and theridge increases when moving along the ridge from the entry side towardthe exit side for at least a portion of the ridge. In some instances,the closest distance between the ridge and the thermal conductor is atthe entry side of the modulator and the longest distance between theridge and the thermal conductor is at the exit side of the modulator.

In some instances, the device includes a heater positioned on the ridge.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A and FIG. 1B illustrates an optical device having a waveguidethat guides a light signal between a light source and a modulator. FIG.1A is a perspective view of the device.

FIG. 1B is a cross section of the device taken along the line labeled Bin FIG. 1A.

FIG. 2A through FIG. 2F illustrate construction of a modulator that issuitable for use as the modulator of FIG. 1A. FIG. 2A is a topview ofthe portion of the optical device shown in FIG. 1A that includes anoptical modulator.

FIG. 2B is a cross-section of the optical device shown in FIG. 2A takenalong the line labeled B.

FIG. 2C is a cross-section of the optical device shown in FIG. 2A takenalong the line labeled C.

FIG. 2D is a cross-section of the optical device shown in FIG. 2A takenalong the line labeled D.

FIG. 2E is a cross-section of the optical device shown in FIG. 2A takenalong the line labeled E.

FIG. 2F is another embodiment of the cross section illustrated in FIG.2E.

FIG. 2G is another embodiment of the cross section illustrated in FIG.2E.

FIG. 3 is a cross section of an embodiment of a modulator having areduced sensitivity to the thickness of the slab regions on opposingsides of a waveguide.

FIG. 4A through FIG. 4C illustrate a localized heater in conjunctionwith a modulator. FIG. 4A is a topview of the portion of the device thatincludes the modulator.

FIG. 4B is a cross section of the modulator shown in FIG. 4A taken alongthe line labeled B in FIG. 4A.

FIG. 4C is a cross section of the modulator shown in FIG. 4A taken alongthe longitudinal axis of the waveguide.

FIG. 5A is a cross section of a portion of a device that includes aheater on a modulator. The heater is positioned over the top and lateralsides of the modulator.

FIG. 5B is a cross section of a portion of a device that includes aheater on a modulator.

FIG. 6A and FIG. 6B illustrate the device of FIG. 4A through FIG. 4C incombination with the modulator of FIG. 2E. FIG. 6A is a cross section ofthe device taken through the modulator.

FIG. 6B is a cross section of the device taken along the length of thewaveguide.

FIG. 7A is a topview of an optical device that includes a modulator,heater, and thermal conductors. The modulator modulates light signalguided through a ridge that includes an electro-absorption medium. Thethermal conductors have an active edge adjacent to the ridge. Thedistance between the active edge of the thermal conductors and the ridgechanges along the length of the modulator.

FIG. 7B is a topview of the optical device shown in FIG. 7A but with theactive edge of the thermal conductors terminating before extending pastthe entry side of the modulator.

FIG. 7C is a topview of the optical device shown in FIG. 7A but with thethermal conductors extending over an input side of theelectro-absorption medium.

FIG. 7D is a topview of the optical device shown in FIG. 7C where thethermal conductors have an active edge with a curved portion.

FIG. 7E is a topview of the optical device shown in FIG. 7C but with thedistances between the active edge and the ridge labeled.

FIG. 7F is an embodiment of the device shown in FIG. 2D where differentportions of a waveguide intersect at an angle less than 180°.

DESCRIPTION

An optical device has an optical modulator positioned on a base. Themodulator includes a ridge extending upward from the base. The ridgeincludes an electro-absorption medium through which light signals areguided. A thermal conductor is positioned so as to conduct thermalenergy away from the ridge. The distance between the thermal conductorand the ridge changes for at least a portion of the ridge. The distancecan be changed so as to reduce hot spots in the modulator. For instance,most of the heat is generated at the entry to the modulator as a resultof increased light absorption near the modulator entry. As a result, thethermal conductor can be closest to the ridge at the modulator entry andthen move away from the ridge as the ridge moves toward the exit of themodulator. Accordingly, the thermal conductor increases the uniformityof the temperature distribution along the ridge. The increasedtemperature uniformity allows higher power levels to be used with themodulator.

In some instances, the optical device includes a localized heaterpositioned on at least a portion of the electro-absorption medium thatis included in the modulator. The heater can be positioned on top of theridge of electro-absorption medium. Electronics can operate the heatersuch that the modulator provides efficient modulation despite thetemperature of the source of the light signal being anywhere in the fulloperational temperature range of the device. Placing the heater on theridge rather than spaced apart from the ridge provides a more directheat transfer to the modulator and accordingly reduces the energyrequirements of the heater. For instance, simulation results have shownthat maximum power usage of only 54-108 mW per heater can be achieved.It may be possible to achieve this same result by controlling thetemperature of the entire device through the use of temperature controlsystems such as thermo-electric coolers (TEC). However, thesetemperature control systems add cost and complexity to the device at thepoint of fabrication. Further, these temperature control system haveundesirably large power requirements and are accordingly associated withongoing operation costs. As a result, the localized heater can reducethe costs and power requirements associated with the device.

The variable distance between the thermal conductor and the ridge canalso enhance the heater performance. As discussed above, the thermalconductor can move away from ridge as the ridge approaches the exit sideof the modulator. The increased distance results in a less efficientconduction of heat away from the ridge at locations downstream of theentry side. The increased heat retention at this location increases theheater efficiency at this location. Accordingly, the heater canefficiently bring the temperature at this portion of the ridge intoagreement with the temperature near the entry side of the modulator andfurther increase the uniformity of the temperature along the length ofmodulator.

FIG. 1A and FIG. 1B illustrate an optical device having a waveguide thatguides a light signal between a light source 8 and a modulator 9. FIG.1A is a perspective view of the device. FIG. 1B is a cross section ofthe device taken along the line labeled B in FIG. 1A. FIG. 1A and FIG.1B do not show details of either the light source 8 or the modulator butillustrates the relationship between these components and the waveguide.

The device is within the class of optical devices known as planaroptical devices. These devices typically include one or more waveguidesimmobilized relative to a substrate or a base. The direction ofpropagation of light signals along the waveguides is generally parallelto a plane of the device. Examples of the plane of the device includethe top side of the base, the bottom side of the base, the top side ofthe substrate, and/or the bottom side of the substrate.

The illustrated device includes lateral sides 10 (or edges) extendingfrom a top side 12 to a bottom side 14. The propagation direction oflight signals along the length of the waveguides on a planar opticaldevice generally extends through the lateral sides 10 of the device. Thetop side 12 and the bottom side 14 of the device are non-lateral sides.

The device includes one or more waveguides 16 that carry light signalsto and/or from optical components 17. Examples of optical components 17that can be included on the device include, but are not limited to, oneor more components selected from a group consisting of facets throughwhich light signals can enter and/or exit a waveguide, entry/exit portsthrough which light signals can enter and/or exit a waveguide from aboveor below the device, multiplexers for combining multiple light signalsonto a single waveguide, demultiplexers for separating multiple lightsignals such that different light signals are received on differentwaveguides, optical couplers, optical switches, lasers that act as asource of a light signal, amplifiers for amplifying the intensity of alight signal, attenuators for attenuating the intensity of a lightsignal, modulators for modulating a signal onto a light signal,detectors that convert a light signal to an electrical signal, and viasthat provide an optical pathway for a light signal traveling through thedevice from the bottom side 14 of the device to the top side 12 of thedevice. Additionally, the device can optionally, include electricalcomponents. For instance, the device can include electrical connectionsfor applying a potential or current to a waveguide and/or forcontrolling other components on the optical device.

A portion of the waveguide includes a first structure where a portion ofthe waveguide 16 is defined in a light-transmitting medium 18 positionedon a base 20. For instance, a portion of the waveguide 16 is partiallydefined by a ridge 22 extending upward from a slab region of thelight-transmitting medium as shown in FIG. 1B. In some instances, thetop of the slab region is defined by the bottom of trenches 24 extendingpartially into the light-transmitting medium 18 or through thelight-transmitting medium 18. Suitable light-transmitting media include,but are not limited to, silicon, polymers, silica, SiN, GaAs, InP andLiNbO₃.

The portion of the base 20 adjacent to the light-transmitting medium 18is configured to reflect light signals from the waveguide 16 back intothe waveguide 16 in order to constrain light signals in the waveguide16. For instance, the portion of the base 20 adjacent to thelight-transmitting medium 18 can be a light insulator 28 with a lowerindex of refraction than the light-transmitting medium 18. The drop inthe index of refraction can cause reflection of a light signal from thelight-transmitting medium 18 back into the light-transmitting medium 18.The base 20 can include the light insulator 28 positioned on a substrate29. As will become evident below, the substrate 29 can be configured totransmit light signals. For instance, the substrate 29 can beconstructed of a light-transmitting medium 18 that is different from thelight-transmitting medium 18 or the same as the light-transmittingmedium 18. In one example, the device is constructed on asilicon-on-insulator wafer. A silicon-on-insulator wafer includes asilicon layer that serves as the light-transmitting medium 18. Thesilicon-on-insulator wafer also includes a layer of silica positioned ona silicon substrate. The layer of silica can serving as the lightinsulator 28 and the silicon substrate can serve as the substrate 29.

Although the light source 8 is shown positioned centrally on the device,the light source 8 can be positioned at the edge of the device. Thelight source 8 can be any type of light source including light sourcesthat convert electrical energy into light. Examples of suitable lightsources include, but are not limited to, a semiconductor laser, and asemiconductor amplifier such as a reflection semiconducting opticalamplifier (RSOA). Examples of suitable lasers include, but are notlimited to, Fabry-Perot lasers, Distributed Bragg Reflector lasers (DBRlasers), Distributed FeedBack lasers (DFB lasers), external cavitylasers (ECLs). A variety of suitable lasers and laser constructions aredisclosed in light source applications including U.S. patent applicationSer. No. 13/385,774, filed on Mar. 5, 2012, and entitled “Integration ofComponents on Optical Device;” U.S. patent application Ser. No.14/048,685, filed on Oct. 8, 2013, and entitled “Use of Common ActiveMaterials in Optical Components;” U.S. Provisional Patent ApplicationSer. No. 61/825,501, filed on May 20, 2013, and entitled “Reducing PowerRequirements for Optical Links;” U.S. patent application Ser. No.13/694,047, filed on Oct. 22, 2012, and entitled “Wafer Level Testing ofOptical Components;” U.S. patent application Ser. No. 13/506,629, filedon May 2, 2012, and entitled “Integration of Laser into OpticalPlatform;” U.S. patent application Ser. No. 13/573,892, filed on Oct.12, 2012, and entitled “Reduction of Mode Hopping in a Laser Cavity;”U.S. patent application Ser. No. 13/317,340, filed on Oct. 14, 2011, andentitled “Gain Medium Providing Laser and Amplifier Functionality toOptical Device;” U.S. patent application Ser. No. 13/385,275, filed onFeb. 9, 2012, and entitled “Laser Combining Light Signals from MultipleLaser Cavities;” each of which is incorporated herein in its entirety.The light source 8 can be constructed as disclosed in any one or more ofthe light source applications and/or can be interfaced with the deviceas disclosed in any one or more of the light source applications. Othersuitable light sources include interdevice waveguides that carry a lightsignal to the device from another device such as an optical fiber. Avariety of interfaces between an optical fiber and a device constructedaccording to FIG. 1A and FIG. 1B are disclosed in fiber interfacepatents applications including U.S. patent application Ser. No.12/228,007, filed on Nov. 14, 2008, and entitled “Optical System HavingOptical Fiber Mounted to Optical Device,” now abandoned; and U.S. patentapplication Ser. No. 12/148,784, filed on Apr. 21, 2008, entitled“Transfer of Light Signals Between Optical Fiber and System UsingOptical Devices with Optical Vias,” and issued as U.S. Pat. No.8,090,231; each of which is incorporated herein in its entirety. Thelight source 8 can an optical fiber interfaced with a device asdisclosed in any one or more of the fiber interface patentsapplications. In some instances, the device does not include a lightsource. For instance, the waveguide can terminate at a facet located ator near the perimeter of the device and a light signal traveling throughair can then be injected into the waveguide through the facet.Accordingly, the light source is optional.

FIG. 2A through FIG. 2E illustrate construction of a modulator that issuitable for use as the modulator of FIG. 1A. FIG. 2A is a topview ofthe portion of the optical device shown in FIG. 1A that includes anoptical modulator. FIG. 2B is a cross-section of the optical deviceshown in FIG. 2A taken along the line labeled B. FIG. 2C is across-section of the optical device shown in FIG. 2A taken along theline labeled C. FIG. 2D is a cross-section of the optical device shownin FIG. 2A taken along the line labeled D. FIG. 2E is a cross-section ofthe optical device shown in FIG. 2A taken along the line labeled E.

Recesses 25 (FIG. 2A) extend into the slab regions such that the ridge22 is positioned between recesses 25. The recesses 25 can extend partway into the light-transmitting medium 18. As is evident from FIG. 2B,the recesses 25 can be spaced apart from the ridge 22. As a result, aportion of the waveguide 16 includes a second structure where an upperportion of the waveguide 16 is partially defined by the ridge 22extending upward from the slab region and a lower portion of thewaveguide is partially defined by recesses 25 extending into the slabregions and spaced apart from the ridge.

As shown in FIG. 2C, the recesses 25 can approach the ridge 22 such thatthe sides of the ridge 22 and the sides of the recesses 25 combine intoa single surface 26. As a result, a portion of a waveguide includes athird structure where the waveguide is partially defined by the surface26.

As is evident in FIG. 2A, a portion of the waveguide 16 includes anelectro-absorption medium 27. The electro-absorption medium 27 isconfigured to receive the light signals from a portion of the waveguidehaving the third structure and to guide the received light signalsthrough the electro-absorption medium 27 such that the light signals areat least partially constrained by a ridge of the electro-absorptionmedium 27. In the illustrated embodiment, the electro-absorption medium27 is configured to guide the received light signals such that they arereceived at another portion of the waveguide having the third structure.Other arrangements are possible. For instance, the electro-absorptionmedium 27 may terminate the waveguide.

In FIG. 2D, a ridge 22 of electro-absorption medium 27 extends upwardfrom a slab region of the electro-absorption medium 27. Accordingly, aportion of a waveguide includes a fourth structure configured to guidethe received light signal through the electro-absorption medium 27. Thisportion of the waveguide is partially defined by the top and lateralsides of the electro-absorption medium 27. The slab regions of theelectro-absorption medium 27 and the ridge 22 of the electro-absorptionmedium 27 are both positioned on a seed portion 34 of thelight-transmitting medium 18. As a result, the seed portion 34 of thelight-transmitting medium 18 is between the electro-absorption medium 27and the base 20. In some instances, when the light signal travels fromthe light-transmitting medium into the electro-absorption medium 27, aportion of the light signal enters the seed portion 34 of thelight-transmitting medium 18 and another portion of the light signalenters the electro-absorption medium 27. As described above, theelectro-absorption medium 27 can be grown on the seed portion of thelight-transmitting medium 18. The seed layer is optional. For instance,the electro-absorption medium 27 can be grown or otherwise formeddirectly on the seed portion of the light-transmitting medium 18

As is evident in FIG. 2A, there is an interface between each facet ofthe electro-absorption medium 27 and a facet of the light-transmittingmedium 18. The interface can have an angle that is non-perpendicularrelative to the direction of propagation of light signals through thewaveguide 16 at the interface. In some instances, the interface issubstantially perpendicular relative to the base 20 while beingnon-perpendicular relative to the direction of propagation. Thenon-perpendicularity of the interface reduces the effects of backreflection. Suitable angles for the interface relative to the directionof propagation include but are not limited to, angles between 80° and89°, and angles between 80° and 85°. The waveguide 16 is shown in FIG.2A as being straight across the interface of the electro-absorptionmedium 27 and a facet of the light-transmitting medium 18; however, thewaveguide 16 can be bent at one or both of these interfaces so as tocompensate for refraction and/or to reduce optical loss due torefraction.

The optical device includes a modulator. In order to simplify FIG. 2A,only a portion of modulator features are shown in FIG. 2A. However, themodulator construction is evident from other illustrations such as FIG.2E. The modulator of FIG. 2E is constructed on the portion of thewaveguide having a fourth structure constructed according to FIG. 2D.The interfaces between different materials are illustrated with solidlines. The modulator is configured to apply an electric field to theelectro-absorption medium 27 in order to phase and/or intensity modulatethe light signals received by the modulator.

A ridge 22 of the electro-absorption medium 27 extends upward from aslab region of the electro-absorption medium 27. Doped regions 40 areboth in the slab regions of the electro-absorption medium 27 and also inthe ridge of the electro-absorption medium 27. Dashed line in FIG. 2Eillustrate the perimeter of portions of the doped regions 40. The dashedlines are used to prevent doped regions from being confused withinterfaces between different materials. The doped regions 40 of theelectro-absorption medium 27 are positioned on the lateral sides of theridge 22 of the electro-absorption medium 27. In some instances, each ofthe doped regions 40 extends up to the top side of theelectro-absorption medium 27 as shown in FIG. 2E. Additionally, thedoped regions 40 extend away from the ridge 22 into the slab region ofthe electro-absorption medium 27. The transition of a doped region 40from the ridge 22 of the electro-absorption medium 27 into the slabregion of the electro-absorption medium 27 can be continuous andunbroken as shown in FIG. 2E. The concentration of dopant in a dopedregion 40 can be uniform, substantially uniform, or can have variationsthat result from limitations of the processes used to form the dopedregions.

Each of the doped regions 40 can be an N-type doped region or a P-typedoped region. For instance, each of the N-type doped regions can includean N-type dopant and each of the P-type doped regions can include aP-type dopant. In some instances, the electro-absorption medium 27includes a doped region 40 that is an N-type doped region and a dopedregion 40 that is a P-type doped region. The separation between thedoped regions 40 in the electro-absorption medium 27 results in theformation of PIN (p-type region-insulator-n-type region) junction in themodulator.

In the electro-absorption medium 27, suitable dopants for N-type regionsinclude, but are not limited to, phosphorus and/or arsenic. Suitabledopants for P-type regions include, but are not limited to, boron. Thedoped regions 40 are doped so as to be electrically conducting. Asuitable concentration for the P-type dopant in a P-type doped regionincludes, but is not limited to, concentrations greater than 1×10¹⁵cm⁻³, 1×10¹⁷ cm⁻³, or 1×10¹⁹ cm⁻³, and/or less than 1×10¹⁷ cm⁻³, 1×10¹⁹cm⁻³, or 1×10²¹ cm⁻³. A suitable concentration for the N-type dopant inan N-type doped region includes, but is not limited to, concentrationsgreater than 1×10¹⁵ cm⁻³, 1×10¹⁷ cm⁻³, or 1×10¹⁹ cm⁻³, and/or less than1×10¹⁷ cm⁻³, 1×10¹⁹ cm⁻³, or 1×10²¹ cm⁻³.

Conductors 43 are each positioned on a slab region. In the illustratedembodiment, at least a portion of each conductor is positioned on theslab regions of the electro-absorption medium 27. In FIG. 2A dashedlines are used to represent the perimeter of the electro-absorptionmedium 27 under the conductors 43 and also to illustrate the perimeterof the recesses 25 under the conductors 43. The conductors can bethermally conductive and, in some instances, are also electricallyconducting.

In the illustrated example, the conductors 43 extend from over theelectro-absorption medium 27 to a position over the light-transmittingmedium 18. For instance, a first portion of each conductor 43 ispositioned such that the electro-absorption medium 27 is between theconductor 43 and the base 20 and a second portion of the conductor 43 ispositioned such that the electro-absorption medium 27 is not between theconductor 43 and the base 20. In some instances, the first portion ofthe conductor 43 is arranged such that a line can be drawn that isperpendicular to a surface of the conductor 43 and also extends throughthe electro-absorption medium 27 and in the second portion of theconductor 43, a line can be drawn that is perpendicular to same surfaceof the conductor 43 while extending through the light-transmittingmedium 18 but not through the electro-absorption medium 27. Although asecond portion of a conductor 43 can be positioned in a recess 25, atleast a portion of the second portion of a conductor 43 can be locatedoutside of the recess as is evident from FIG. 2A, FIG. 2D, and FIG. 2E.

One or more claddings 41 are optionally positioned between theconductors 43 and the base 20. In some instances, a cladding 41 isbetween one of the conductors 43 and a doped region 40. The claddings 41can directly contact the light-transmitting medium 18. A cladding thatcontacts the light-transmitting medium 18 included in the waveguidepreferably has a lower index of refraction than the light-transmittingmedium 18. When the light-transmitting medium 18 is silicon, suitablecladdings include, but are not limited to, polymers, silica, SiN andLiNbO₃.

A material layer 42 can be located over the conductors 43. As will bediscussed in more detail below, the material layer 42 can beelectrically insulating. Suitable material layers 42 include, but arenot limited to, silica and silicon nitride. In some instances, a singlelayer of material can serve as both a cladding 41 and a material layer42. Although the material layer 42 is shown as a single layer ofmaterial, the material layer 42 can include or consist of multiplelayers of material 41.

The conductors 43 can be in electrical communication with a dopedregion. For instance, in some instances, the conductors are in directphysical contact with doped region 40 at a contact 44. As an example,the conductors can extend through an opening in the material layer 41into contact with the underlying doped region. The conductors can alsobe in electrical communication with contact pads 45. For instance,openings in the one or more layers of material 41 can expose one or moreportions of the conductors 43 that act as contact pads 45. The contactpads 45 can serve as electrical contacts and/or contact pads for theelectronics. Accordingly, when the conductors 43 are electricallyconducting, one or more of the conductors 43 can provide electricalcommunication between the electronics and a doped region 40. In someinstances, the conductors 43 are electrically conducting and the dopedregions 40 are doped at a concentration that allows the doped regions 40to act as electrical conductors. Accordingly, the conductors 43 anddoped regions 40 provide an electrical pathway between the electronicsand the ridge of electro-absorption medium 27. As a result, theelectronics can apply energy to the electrical conductors 43 in order toapply an electric field to the electro-absorption medium 27. The regionof the light-transmitting medium or electro-absorption medium betweenthe doped regions can be undoped or lightly doped as long as the dopingis insufficient for the doped material to act as an electrical conductorthat electrically shorts the modulator.

During operation of the modulators of FIG. 1A through FIG. 2E,electronics 47 (FIG. 1A) can be employed to apply electrical energy tothe electrical conductors 43 so as to form an electrical field in theelectro-absorption medium 27. For instance, the electronics can form avoltage differential between the doped regions that act as a source ofthe electrical field in the gain medium. The electrical field can beformed without generating a significant electrical current through theelectro-absorption medium 27. The electro-absorption medium 27 can be amedium in which the Franz-Keldysh effect occurs in response to theapplication of the electrical field. The Franz-Keldysh effect is achange in optical absorption and optical phase by an electro-absorptionmedium 27. For instance, the Franz-Keldysh effect allows an electron ina valence band to be excited into a conduction band by absorbing aphoton even though the energy of the photon is below the band gap. Toutilize the Franz-Keldysh effect the active region can have a slightlylarger bandgap energy than the photon energy of the light to bemodulated. The application of the field lowers the absorption edge viathe Franz-Keldysh effect and makes absorption possible. The hole andelectron carrier wavefunctions overlap once the field is applied andthus generation of an electron-hole pair is made possible. As a result,the electro-absorption medium 27 can absorb light signals received bythe electro-absorption medium 27 and increasing the electrical fieldincreases the amount of light absorbed by the electro-absorption medium27. Accordingly, the electronics can tune the electrical field so as totune the amount of light absorbed by the electro-absorption medium 27.As a result, the electronics can intensity modulate the electrical fieldin order to modulate the light signal. Additionally, the electricalfield needed to take advantage of the Franz-Keldysh effect generallydoes not involve generation of free carriers by the electric field.

Heat is generated as a result of the electro-absorption medium 27absorbing light during the operation of the modulator. The label of“light signal direction” is used in FIG. 2A to indicate that thedirection of propagation for light signals during operation of themodulator. The light signal enters the electro-absorption medium 27through an input side of the electro-absorption medium 27 and exits fromthe electro-absorption medium 27 through an output side of theelectro-absorption medium 27. Generally, the light absorption is mostintense where the light signal first interacts with the electricalfield. In general, this occurs at the interface of thelight-transmitting medium 18 and the electro-absorption medium 27. As aresult, light absorption is generally most intense at or near the inputside of the electro-absorption medium 27. The increased light absorptioncan lead to a hot spot in the modulator. When the conductors are thermalconductors, the extension of the conductors 43 from over theelectro-absorption medium 27, across the input side of theelectro-absorption medium 27 to a location over the light-transmittingmedium 18 provides a pathway for the heat generated by the modulator tobe carried away from the modulator and accordingly provides cooling ofthe modulator.

When the conductors 43 provides heat dissipation, the second portion ofthe conductors can have dimensions that exceed the dimensions that arecommonly used for metal traces designed to carry electrical currents onintegrated circuit boards. For instance, one or more of the conductors43 can cover a portion of the device having an area more than 4,000microns, 20,000 microns, or 100,000 microns' and/or a perimeter lengthgreater than 300 microns, 500 microns, or 1000 microns. Additionally oralternately, one or more of the conductors can have a thickness greaterthan 50 picometers, 0.1 micron, or 1 micron. In some instances, the oneor more of the conductors a thickness greater than 50 picometers, 0.1micron, or 1 micron for more than 50% or 85% of the device area coveredby the conductor. Although FIG. 2A illustrates the conductors 43 ashaving a somewhat regular shape, one or more of the conductors can haveirregular shapes and/or patterns.

Although FIG. 2A, FIG. 2D, and FIG. 2E illustrate the conductors 43terminating before contacting the ridge, a portion of each one of theone or more conductors 43 can extend into contact with the ridge asillustrated in FIG. 2F. FIG. 2F is a cross section of the modulatorillustrated in FIG. 2E but with the conductors 43 contacting the ridge.The conductors extend into contact with a vertical portion of the one ormore claddings 41 positioned on the ridge. The one or more claddings 41are optionally arranged so as to keep the contacts 44 spaced apart fromthe ridge.

The conductors 43 can extend up the lateral sides of the ridge asillustrated in FIG. 2G. FIG. 2G is a cross section of the modulatorillustrated in FIG. 2F but with the conductors 43 positioned over thelateral sides of the ridge. In particular, the conductors 43 eachincludes a lateral portion that bends away from a base portion. The baseportion is positioned over the slab regions adjacent to the ridge andthe lateral portions are positioned over the lateral sides of theelectro-absorption medium 27. In FIG. 2G, the lateral portions are shownas being vertical relative to the base 20. Moving the conductors closerto the ridge as illustrated in FIG. 2F and FIG. 2G reduces the distancethat thermal energy generated in the ridge must travel before beingconducted away from the ridge. Accordingly, these arrangements canprovide a more efficient conduction of heat away from the ridge.

Although FIG. 2A, FIG. 2D, and FIG. 2E through FIG. 2G illustrate bothconductors 43 extending beyond the perimeter of the modulator and/or theelectro-absorption medium 27, in some instances, only one of theconductors 43 extends beyond the perimeter of the modulator and/or theelectro-absorption medium 27.

A modulator having a cross section according to FIG. 2E through FIG. 2Gcan be sensitive to the thickness of the slab regions of theelectro-absorption medium 27. For instance, as the thickness of the slabregion increases, the ridge becomes smaller and the electrical fieldformed between the doped regions 40 accordingly fills a smaller portionof the distance between the base 20 and the top of the ridge. Forinstance, the location of the electrical field effectively moves upwardsfrom the base 20. The increased space between the electrical field andthe base 20 can be thought of as increasing the resistance or carrierdiffusion time of the modulator. This increase in resistance and/ordiffusion time decreases the speed of the modulator. Problems also occurwhen these slab regions become undesirably thin. When these slab regionsbecome thin, the doped regions extend down into the light-transmittingmedium 18. This doping of the light-transmitting medium 18 alsodecreases the speed of the modulator.

FIG. 3 presents an embodiment of a modulator having a reducedsensitivity to the thickness of the slab regions. The perimeter ofportions of doped regions shown in FIG. 3 are illustrated with dashedlines to prevent them from being confused with interfaces betweendifferent materials. The interfaces between different materials areillustrated with solid lines. A first doped zone 46 and a second dopedzone 48 combine to form each of the doped regions 40. In some instance,the first doped zone 46 is located in the light-transmitting medium 18but not in the electro-absorption medium 27 and the second doped zone 48is located in the electro-absorption medium 27. The first doped zone 46can contact the second doped zone 48 or can overlap with the seconddoped zone 48. In some instances, the first doped zone 46 and the seconddoped zone 48 overlap and at least a portion of the overlap is locatedin the light-transmitting medium 18. In other instances, the first dopedzone 46 and the second doped zone 48 overlap without any overlap beingpresent in the electro-absorption medium 27.

The first doped zone 46 and the second doped zone 48 included in thesame doped region 40 each includes the same type of dopant. Forinstance, the first doped zone 46 and the second doped zone 48 in ann-type doped region 40 each includes an n-type dopant. The first dopedzone 46 and the second doped zone 48 included in the same doped region40 can have the same dopant concentration or different concentrations.

Although FIG. 3 illustrates the slab regions including theelectro-absorption medium 27, the slab regions of the electro-absorptionmedium 27 may not be present. For instance, the etch that forms the slabregions of the electro-absorption medium 27 may etch all the way throughthe slab regions. In these instances, the first doped zone 46 and thesecond doped zone 48 are both formed in the light-transmitting medium.

Although FIG. 3 shows the first doped zone 46 not extending down to thelight insulator 28, the first doped zone 46 can extend down to the lightinsulator 28 or into the light insulator 28.

Although the modulator is disclosed as having electrical conductors 43in addition to the conductors 43, the electrical conductors 43 areoptional. For instance, the modulator can exclude the electricalconductors 43 and a portion or a region of each conductor 43 can serveas an electrical contact and/or a contact pad. Accordingly, theelectronics can be connected directly to the conductors 43. In someinstances, the conductors 43 do not carry electrical current during theoperation of the modulator. For instance, the device can include otherelectrical pathways (not illustrated) that the electronics can employ tooperate the modulator without driving an electrical current through theconductors 43.

Suitable electro-absorption media 27 for use in the modulator includesemiconductors. However, the light absorption characteristics ofdifferent semiconductors are different. A suitable semiconductor for usewith modulators employed in communications applications includesGe_(1−x)Si_(x) (germanium-silicon) where x is greater than or equal tozero. In some instances, x is less than 0.05, or 0.01. Changing thevariable x can shift the range of wavelengths at which modulation ismost efficient. For instance, when x is zero, the modulator is suitablefor a range of 1610-1640 nm. Increasing the value of x can shift therange of wavelengths to lower values. For instance, an x of about 0.005to 0.01 is suitable for modulating in the c-band (1530-1565 nm).

Suitable materials for the conductors 43 include, but are not limitedto, materials having a thermal conductivity greater than 100, 200, or300 W/(m·K). Examples conductors 43 include or consist of materials suchas metals, epoxies, and dielectrics such as diamond. Particular examplesof a conductor 43 include aluminum, gold, diamond. Aluminum has athermal conductivity of 205 W/m·K and gold has a thermal conductivity of310 W/m·K. Diamond has a thermal conductivity has a thermal conductivityof 1000 W/m·K but is not electrically conducting. The electricalconductors 43 and the conductors 43 can be constructed of the same ordifferent materials. Suitable materials for the electrical conductors 43include, but are not limited to, Al, Ti, Ni, Cu, and Au.

In some instances, the above modulators include a localized heaterconfigured to heat all or a portion of the modulator. When a localizedheaters is used with the above devices, FIG. 2A through FIG. 3 canillustrate the parts located under the heater. FIG. 4A through FIG. 4Cillustrate the localized heater in conjunction with a modulator. FIG. 4Ais a topview of the portion of the device that includes the modulator.FIG. 4B is a cross section of the modulator shown in FIG. 4A taken alongthe line labeled B in FIG. 4A. FIG. 4C is a cross section of themodulator shown in FIG. 4A taken along the longitudinal axis of thewaveguide 16. The details of the modulator are not illustrated. Forinstance, the modulator is generally labeled 9 in FIG. 4A through FIG.4C with the electrical conductors 43 being labeled in FIG. 4A.

The heater 50 is on the ridge 22 such that the modulator 9 is positionedbetween the heater 50 and the base. The one or more material layers 42can optionally be positioned between the heater and the ridge. Forinstance, the heater 50 can be located on a material layer 42 thatelectrically insulates the heater 50 from the underlying layers and/oroptically isolates the underlying layers from the heater 50. Thematerial layer 42 can be positioned between the heater and the ridge 22.Suitable material layers 42 include, but are not limited to, silica andsilicon nitride. A material layer 42 with a higher thermal conductivitymay be preferred in or to provide a pathway from heat to travel from theheater to the modulator. Accordingly, material layers 42 that arethinner and/or have a higher thermal conductivity may be desired. Insome instances, the material layer 42 has a thermal conductivity above10 W/mK.

The one or more claddings 41 are optionally positioned between thewaveguide 16 and the material layer 42 and/or between the waveguide 16and the heater 50. At least one of the claddings 41 can directly contactthe light-transmitting medium 18. A cladding 41 that contactslight-transmitting medium 18 can have a lower index of refraction thanthe light-transmitting medium 18. When the light-transmitting medium 18is silicon, suitable claddings include, but are not limited to,polymers, silica, SiN and LiNbO₃. In some instances, a single layer ofmaterial can serve as both a cladding 41 and a material layer 42.

Conductors 56 are positioned so as to provide electrical communicationbetween the heater 50 and contact pads 58. The conductors 56 and contactpads 58 can be electrically conducting. The electronics 47 can applyelectrical energy to the contact pads 58 so as to deliver electricalenergy to the heater 50 and can accordingly operate the heater so theheater 50 generates heat. The location of the heater on the ridge 22allows the generated heat to elevate the temperature of the ridgethrough a mechanism such as conduction.

In some instances, the heater 50 is an “electrical resistance heater.”For instance, the heater 50 can include or consist of an electricallyconducting layer 60 that serves as a resistor. An example of a suitableresistor is a trace that includes or consists of a metal, metal alloy.Examples of heaters include or consist of titanium traces, tungstentitanium traces, titanium nitride traces, and/or nichrome traces. Duringoperation of the device, the electronics 47 can drive sufficientelectrical current through the electrically conducting layer 60 to causethe electrically conducting layer 60 to generate the heat that isconducted to the modulator. The conductors 56 can include or consist ofan electrically conductive layer 62 and can be arranged such that theelectrical current flows parallel or substantially parallel to the ridge22 or the direction of light signal propagation through the ridge. As aresult, the length of the ridge 22 that is heated by the heater can beincreased merely by increasing the length of the resistor.

The electrically conducting layer 60 can have a higher resistance/lengththan the electrically conductive layers 62 in order to stop or reducegeneration of heat by the conductors 56. This can be achieved by usingdifferent materials and/or dimensions for the electrically conductivelayer 62 and the conducting layer 60. For instance, the electricallyconductive layer 62 can be aluminum while the conducting layer 60 thatserves as the heater is titanium. Titanium has a specific electricalresistance of about 55 μohm-cm while aluminum has a specific electricalresistance of about 2.7 μohm-cm. As a result, the conductors 56 andconducting layer 60 can have similar cross sectional dimensions and anelectrical current can be driven through the conductors 56 andconducting layer such that heat is generated at the conducting layerwithout undesirable levels of heat being generated by the conductors 56.Alternately, the conductors 56 can have larger cross section dimensionsthan the heater in order to further reduce heat generation by theconductors 56

In some instances, the conductors 56 include a conducting layer 60 fromthe heater 50 in addition the conductive layer 62 as is evident in FIG.4B. In these instances, the conductive layer 62 can be more conductiveand/or have larger dimensions than the conducting layer 60 in order toreduce generation of heat by the conductor 56. When the conductors 56include the conducting layer 60 and the conductive layer 62, theconductors 56 and heater can be formed by forming a first layer of thematerial for the conducting layer and then forming a second layer ofmaterial for the conductive layer over the first layer. Suitable methodsfor forming the first layer and the second layer on the device include,but are not limited to, sputtering, evaporation, PECVD and LPCVD. Thefirst layer and the second layer can then be patterned so as to form theconductors and heater on the device. Suitable methods for patterninginclude, but are not limited to, etching in the presence of one or moremasks. The portion of the second layer over the heater 50 can then beremoved to provide the configuration of conducting layer and conductivelayer shown in FIG. 4A and FIG. 4B. Suitable methods for removing theportion of the second layer include, but are not limited to, etching inthe presence of a mask. Although the electrically conducting layer 60and the electrically conductive layers 62 are disclosed as a singlelayer of material, either or both of the electrically conducting layer60 and the electrically conductive layers 62 can include or consist ofone or more different layers of material.

A suitable ratio for the specific electrical resistance of theconducting layer 60: conductive layer 62 is greater than 5:1, 10:1, or50:1.

FIG. 4A through FIG. 4C illustrate the heater 50 as being positioned onthe top of the electro-absorption medium or on top of the ridge 22.Additionally or alternately, the heater can be positioned on one or morelateral sides of the electro-absorption medium or on one or more lateralsides of the ridge 22. For instance, FIG. 5A is a cross section of thedevice such as the cross section of FIG. 4B. FIG. 5A illustrates theheater positioned on both the top and lateral sides of the ridge 22. Asa result, the heater is positioned on both the top and lateral sides ofthe electro-absorption medium 27. In some instances, the heater 50 ispositioned on one or more of the lateral sides of the electro-absorptionmedium 27 without being positioned on the top of the electro-absorptionmedium 27 and/or on one or more of the lateral sides of the ridge 22without being positioned on the top of the ridge 22. The heater does notextend down to the base of the ridge but can extend all the way to thebase of the ridge.

The heater 50 can extend away from the ridge 22 such that the heater 50is positioned over the slab regions. For instance, FIG. 5B is a crosssection of the modulator where the heater is positioned on the ridge ofthe electro-absorption medium 27, extends down to the base of the ridge22, and extends away from the base of the ridge 22 on the slab regions.The distance that the heater extends away from the ridge is labeled E inFIG. 5B. The distance is equal to the distance between the edge of theheater and the portion of the heater on the lateral side of the ridge22. Increasing the distance that the heater extends away from the ridgecan reduce the degree of localized heating and can increase the powerrequirements for the device. In some instances, the distance that theheater extends away from the ridge is less than 2 μm, 1 μm, or 0.5 μmand can be 0 μm. The bottom or lower side of the heater 50 is betweenthe top (or upper side) of the heater 50 and the modulator 9 and/or theelectro-absorption medium 27. In some instances, the heater 50 isarranged such that the bottom (or lower side) of the heater 50 does notcontact the device at a location that is more than 2 μm, 200 μm, or 500μm away from a lateral side of the ridge and/or an edge of the heater isnot located more than 2 μm, 200 μm, or 500 μm away from the nearestlateral side of the ridge. In other words, no portion of the heaterthrough which heat travels to the device is located more than 2 μm, 200μm, or 500 μm away from the nearest lateral side of the ridge or theheater is not positioned over a location that is more than 2 μm, 200 μm,or 500 μm away from the nearest lateral side of the ridge.

In FIG. 4A through FIG. 5B, the bottom (lower side) of the heater 50 isbetween the top of the heater 50 and the modulator 9 and/or theelectro-absorption medium 27. Moving the bottom of the heater 50 closerto the electro-absorption medium 27 and/or the ridge 22 reduces thedistance over which the generated heat must be conducted in order toelevate the temperature of the modulator and can accordingly reduce theamount of heat that must be generated in order to achieve a particulartemperature within the modulator. Reducing the thickness of the one ormore layers of material between the bottom of the heater and theelectro-absorption medium 27 can move the bottom of the heater 50 closerto the electro-absorption medium 27. For instance, reducing thethickness of the one or more claddings 41 and the one or more materiallayers 42 can move the bottom of the heater 50 closer to theelectro-absorption medium 27. In some instances, all or a portion of thebottom of the heater 50 is within 0.5, 1, or 2 μm of theelectro-absorption medium 27.

The details of the modulator construction are not illustrated in FIG. 4Athrough FIG. 5B; however, the modulator can have a variety ofconstructions including, but not limited to, the constructions of FIG.2E through FIG. 2G or FIG. 3. In order to illustrate this concept, FIG.6A and FIG. 6B illustrate the device of FIG. 4A through FIG. 4C incombination with the modulator of FIG. 2E. FIG. 6A is a cross section ofthe device taken through the modulator. FIG. 6B is a cross section ofthe device taken along the length of the waveguide. The heater 50 ispositioned over at least a portion of the electro-absorption medium 27that is included in the modulator such that the electro-absorptionmedium 27 is located between the heater 50 and the base. FIG. 6B showsthat the heater 50 does not extend beyond the perimeter of theelectro-absorption medium 27; however, one or both ends of the heatercan extend beyond the perimeter of the electro-absorption medium 27.

As is evident in FIG. 6A, a protective layer 64 can optionally be formedover the above devices. In some instances, the protective layer 64 canhave a thermal conductivity that is less than the thermal conductivityof the one or more claddings 41 and/or the one or more material layers42. The reduced thermal conductivity of the protective layer 64 causesheat generated by the heater to be directed toward the modulator and canaccordingly reduce the energy requirements of the heater as well asreduce thermal cross talk. Suitable protective layers include, but areno limited to, silica, silicon nitride, and aluminum oxide. Although theprotective layer is disclosed as a single layer of material, theprotective layer can be constructed of multiple layers of material. Insome instances, one, two or three layers of the protective layer have athermal conductivity greater than 0.75 WK/m, 1.0 WK/m, or 1.25 WK/m. Theprotective layer is not illustrated in FIG. 6B.

FIG. 7A through FIG. 7E are presented to illustrate the relationshipbetween the modulator, heater 50, and conductors 43 disclosed above.FIG. 7A through FIG. 7E are each a topview of the device; however, alimited number of the device features are shown. For instance, the onlyportion of the modulator that is illustrated is the field sources 66.The field sources 66 are the source of the electrical field in and/orunder the ridge of electro-absorption medium. Accordingly, the dopedregions 40 disclosed above can serve as the field sources; however,other materials such as metals can serve as field sources. Additionally,the material layer 42 and one or more claddings 41 are not illustratedin FIG. 7A through FIG. 7E. Additionally dashed lines are used to showthe perimeter of features that are located under other features of thedevice. The label of “light signal direction” is used in FIG. 7A throughFIG. 7E to indicate that the direction of propagation for light signalsduring operation of the modulator although it may be possible for lightsignals to travel in the opposing direction during operation of themodulator. The light signal enters the electro-absorption medium 27through an input side of the electro-absorption medium 27 and exits fromthe electro-absorption medium 27 through an output side of theelectro-absorption medium 27.

The conductors 43 include an active edge 68 that is the edge of theconductors 43 located closest to the ridge 22. The modulation of thelight signal primarily occurs where the electrical field is formed inthe electro-absorption medium 27. Accordingly, the field sources 66define an entry side 70 and an exit side 72 for the modulator. Forinstance, the doped regions 40 disclosed above define an entry side 70and an exit side 72 for the modulator. The conductors 43 are constructedsuch that when moving from the entry side 70 to the exit side 72 for atleast a portion of the conductor, the active edge 68 moves away from theridge of the electro-absorption medium. More specifically, the distancebetween the leading edge 68 and the ridge 22 increases moving from theentry side to the exit side for at least a portion of the leading edgewhere the distance is measured perpendicular to a lateral side of theridge 22. As noted above, heat is generated as a result of theelectro-absorption medium 27 absorbing light during the operation of themodulator. As noted above, the light absorption is generally mostintense where the light signal first interacts with the electricalfield. As a result, the heat is generally generated most intensely nearthe entry side 70. The active edge 68 being closer to the ridge 22 atthe entry side 70 results in a more efficient conduction of heat awayfrom the ridge 22 near the entry side 70 than occurs further downstreamof the entry side. The increased efficiency of heat conduction near theentry side 70 reduces the formation of hot spots at or near the entryside 70 of the modulator. The active edge 68 becoming further from theridge downstream of the entry side results in a less efficientconduction of heat away from the ridge at locations downstream of theentry side 70. As a result, the heat provided by the heater 50 remainsin the ridge 22 longer at locations downstream of the entry side 70where less heat is generated as a result of absorption. By conductingheat away from the location where heat is generated by absorption butallowing heat from the heater to remain in the waveguide at locationswhere less heat is generated by absorption, the uniformity of thetemperature along the length of modulator is increased. Increasing thisuniformity allows the modulator to be operated at an efficienttemperature.

In FIG. 7A, the active edge 68 of the conductors 43 extends past theentry side 70 of the modulator; however, the active edge 68 of theconductors 43 can terminate between the entry side 70 and the exit side72 as is illustrated in FIG. 7B. Extending the active edge 68 of thethermal conductors past the entry side 70 increases the area of theconductor 43 that is available for carrying heat away from the entryside 70 and may be the more desirable arrangement. Although FIG. 7A andFIG. 7B illustrate the active edge 68 of the conductors 43 extendingpast the exit side 72, the active edge 68 of the conductors 43 canterminate between the entry side 70 and the exit side 72 withoutextending past the exit side 72.

One or more of the conductors 43 can extend over the output side of theelectro-absorption medium 27 and/or over the input side of theelectro-absorption medium 27. FIG. 7C illustrates the conductors 43extending over the input side of the electro-absorption medium 27. Forinstance, the active edge 68 extends over the interface between theelectro-absorption medium 27 and a facet of the light-transmittingmedium 18. This arrangement can increase the efficiency at which heat isconducted away from the entry side of the modulator.

The active edge 68 of each conductor illustrated in FIG. 7A through FIG.7C includes two portions that are parallel to the ridge 22 connected bya connecting portion; however, the active edge 68 can have otherconfigurations constructed such that when moving from the entry side 70to the exit side 72 for at least a portion of the conductor, the activeedge 68 moves away from the ridge of the electro-absorption medium. Forinstance, the active edge can have a stair step configuration, one ormore curved regions, and/or other configurations. As an example, FIG. 7Dillustrates a version where a portion of leading edge 68 is curved. Morespecifically, the distance between the leading edge 68 and the ridge 22gradually increases moving from the entry side to the exit side wherethe distance is measured perpendicular to a lateral side of the ridge22.

FIG. 7E is a portion of the device illustrated in FIG. 7C blown up to alarger size. The shortest distance between the ridge 22 and the activeedge 68 is labeled D_(N) and the longest distance between the ridge 22and the active edge 68 is labeled D_(X) where the distances D_(N) andD_(X) are measured perpendicular to the lateral of the ridge. Asillustrated in FIG. 7D, the distance D_(N) can be located at the entryside 70 of the modulator but can be in a different location.Additionally, the distance D_(X) can be located at the exit side 72 ofthe modulator but can be in a different location. A suitable distancefor D_(N) includes but is not limited to distances greater than or equalto 0, 0.01 micron, 0.05 micron and/or less than or equal to 0.5 micron.As is evident from FIG. 2F, FIG. 2G and the associated text, D_(N) canbe limited to the thickness of the one or more cladding layers. Theseone or more cladding layers can have a thickness of about 0.2 microns,0.325 microns, or 0.75 microns. Accordingly, D_(N) can be less than 1micron, 0.5 microns, or even 0.1 microns. In instances, where the one ormore claddings 41 are not used in an embodiment such as FIG. 2F or FIG.2G, D_(N) can be zero. Additionally or alternately, suitable distancefor D_(X) includes but is not limited to distances greater than or equalto 0.02 micron, 0.5 micron, or 1 micron and/or less than or equal to 1.5micron, 5 micron, or 15 micron. Accordingly, in some instances, a ratioof D_(X):D_(N) is greater than or equal to 1.2:1, 2:1, 5:1, 10:1, or20:1. Since D_(N) can be zero, the ratio of D_(X):D_(N) can be infinite.Although FIG. 7D and FIG. 7E essentially uses the embodiment of FIG. 7Cto illustrate D_(N), D_(X), and active edge 68 configurations, thesedistances and active edge 68 configurations can apply to other devicesconstructed as discussed above.

The above illustrations show a straight intersection angle between theportion of the waveguide defined by the light-transmitting medium 18 andthe portion of the waveguide defined by the electro-absorption medium27; however, these different portions of the waveguide can intersect atan angle other than 180°. As an example, FIG. 7F is the topviewillustrated in FIG. 7D but with the portion of the waveguide defined bythe light-transmitting medium 18 and the portion of the waveguidedefined by the electro-absorption medium 27 intersecting at an angleless than 180°. The particular angle at the intersection of thesewaveguide portions can be selected to reduce optical loss. In someinstances, the angle is greater than 150°, 160°, or 170° and/or lessthan 175°, or 180°.

FIG. 7A through FIG. 7F illustrate the heater 50 extending beyond theinput side of the electro-absorption medium 27 and beyond the outputside of the electro-absorption medium 27; however, the heater 50 can beconstructed such that the heater does not extend beyond the input sideof the electro-absorption medium 27 and/or the output side of theelectro-absorption medium 27. As an example, FIG. 4C illustrates aheater that does not extend beyond the input side of theelectro-absorption medium 27 and also does not extend beyond the outputside of the electro-absorption medium 27. Additionally, FIG. 7A throughFIG. 7F illustrate the heater 50 extending from between the entry side70 and the exit side 72 to locations beyond the entry side 70 and theexit side 72 of the modulator; however, the heater 50 can be constructedsuch that the heater is between the entry side 70 and the exit side 72but does not extend beyond the entry side 70 and/or the exit side 72.For instance, it may be desirable to terminate the heater before itextends beyond the entry side of the modulator in order to furtherreduce the level of heat that is generated at the entry side 70 of themodulator.

The modulator of FIG. 4A through FIG. 7F can have constructions otherthan the constructions of FIG. 1A through FIG. 3. Examples of othersuitable modulator constructions can be found in U.S. patent applicationSer. No. 12/653,547, filed on Dec. 15, 2009, entitled “Optical DeviceHaving Modulator Employing Horizontal Electrical Field,” and U.S. patentapplication Ser. No. 13/385,774, filed on Mar. 4, 2012, entitled“Integration of Components on Optical Device,” each of which isincorporated herein in its entirety. U.S. patent application Ser. Nos.12/653,547 and 13/385,774 also provide additional details about thefabrication, structure and operation of these modulators. In someinstances, the modulator is constructed and operated as shown in U.S.patent application Ser. No. 11/146,898; filed on Jun. 7, 2005; entitled“High Speed Optical Phase Modulator,” and now U.S. Pat. No. 7,394,948;or as disclosed in U.S. patent application Ser. No. 11/147,403; filed onJun. 7, 2005; entitled “High Speed Optical Intensity Modulator,” and nowU.S. Pat. No. 7,394,949; or as disclosed in U.S. patent application Ser.No. 12/154,435; filed on May 21, 2008; entitled “High Speed OpticalPhase Modulator,” and now U.S. Pat. No. 7,652,630; or as disclosed inU.S. patent application Ser. No. 12/319,718; filed on Jan. 8, 2009; andentitled “High Speed Optical Modulator;” or as disclosed in U.S. patentapplication Ser. No. 12/928,076; filed on Dec. 1, 2010; and entitled“Ring Resonator with Wavelength Selectivity;” or as disclosed in U.S.patent application Ser. No. 12/228,671, filed on Aug. 13, 2008, andentitled “Electrooptic Silicon Modulator with Enhanced Bandwidth;” or asdisclosed in U.S. patent application Ser. No. 12/660,149, filed on Feb.19, 2010, and entitled “Reducing Optical Loss in Optical Modulator UsingDepletion Region;” each of which is incorporated herein in its entirety.A review of the modulators disclosed in these applications shows thatthe slab regions of the electro-absorption medium 27 are optional. Theheater 50, one or more material layers 42, one or more claddings 41, andconductors 56 can be fabricated using fabrication technologies that areemployed in the fabrication of integrated circuits, optoelectroniccircuits, and/or optical devices. Although all or a portion of the abovemodulators are disclosed in the context of modulators that use theFranz-Keldysh effect, modulators that use other mechanisms to achievemodulation can be employed. Accordingly, the electro-absorption medium27 can represent a different medium or combination of media throughwhich the light signals are guided during modulation.

The device can also include one or more temperature sensors (not shown)that are each positioned to sense the temperature of the modulatorand/or the temperature of a zone adjacent to the modulator. Suitabletemperature sensors include, but are not limited to, thermocouples,thermistors, integrated PN diodes, or other integrated semiconductordevices.

The electronics can adjust the level of electrical energy applied to theheater in response to the output received from the one or moretemperature sensors in a feedback loop. For instance, the electronicscan operate the heater such that the temperature of the heater stays ator above a threshold temperature (T_(th)) during operation of thedevice. For instance, when the electronics determine that thetemperature of the modulator falls below the threshold temperature, theelectronics can apply electrical energy to the heater so as to bring thetemperature of the modulator to or above the threshold temperature.However, when the electronics determine that the temperature of themodulator falls above the threshold temperature, the electronics canrefrain from applying the electrical energy to the heater. As a result,when the electronics determine that the temperature of the modulator isabove the threshold temperature, the temperature of the modulator canfloat in response to the operation of the device in the ambientatmosphere.

The device is configured to operate over an operational ambienttemperature range. For instance, the device should be able to continueoperating when the ambient temperature in which the device is positioned(TA) extends from TL to TH. In some instances, TL is below 0° C., 10°C., or 20° C. and/or TH is greater than 50° C., 70° C., or 80° C. Theoperational ambient temperature range is typically from TL=0° C. toTH=70° C. The operational temperature range is generally defined as partof the specification for the device. In general the operationaltemperature range is designed so the device meets customer requirements.

The width of the band of wavelengths that can be efficiently modulatedby a modulator is the operating bandwidth (OBW) of the modulator. Theoperating bandwidth is generally the length of the band of wavelengthswhere the modulator has low insertion loss and high extinction ratio ata particular temperature. For a Franz Keldysh modulator constructedaccording to FIG. 2E through FIG. 2G, the operating bandwidth (OBW) isgenerally about 35 nm. The operating bandwidth (OBW) for a modulator canbe identified by applying a modulation signal to the modulator andmeasuring the response of the optical signal through the modulator overa range of wavelengths. The range of wavelengths for which the insertionloss and high extinction ratio produce loss of less than 1 dB can serveas the operating bandwidth. In some instances, the range of wavelengthsfor which the insertion loss and high extinction ratio produce loss ofless than 1.5 dB or 2.0 can serve as the operating bandwidth. In someinstances, the operating wavelength range for a modulator is more than25 nm, 30 nm, or 35 nm and/or less than 40 nm, 50 nm, or 60 nm.

The wavelength at the center of the operating bandwidth (OBW) isconsidered the modulation wavelength. The wavelengths that fall withinthe operating bandwidth (OBW) shifts in response to temperature changes;however, the operating bandwidth (OBW) stays constant or substantiallyconstant. As a result, the modulation wavelength is a function oftemperature but the operating bandwidth (OBW) can be approximate asbeing independent of temperature. The rate that the modulationwavelength of the above modulators shifts in response to temperaturechanges (Δλ_(m)) is about 0.76 nm/° C. and the operating bandwidth (OBW)stays substantially constant at about 35 nm.

The most intense wavelength produced by the light source is consideredthe channel wavelength of the light signal produced by the light source.The light source and the modulator are generally configured to operatetogether at a design temperature (TT). For instance, the light sourceand modulator are generally configured such that the modulationwavelength and the channel wavelength are the same at the designtemperature. As a result, the modulator efficiently modulates the outputof the light source at the design temperature. The design temperature isgenerally equal to a common temperature for the ambient environment inwhich the device is positioned. A typical design temperature is 60° C.In some instances, the design temperature serves as the thresholdtemperature (T_(th)).

The channel wavelength and the modulation wavelength at the designtemperature are the design wavelength (λ_(T)). The modulation wavelengthat a particular temperature can be expressed relative to the designwavelength. For instance, the modulation wavelength at a particulartemperature can be expressed as λ_(T)−(TT−T_(m))(Δλ_(m)) where T_(m)represents the temperature of the modulator.

The channel wavelength shifts in response to changes in the temperatureof the light source (T_(LS)). For instance, the channel wavelength shiftrate for a light source (Δλ_(LS)) such as a DFB laser is generally about0.08 nm/° C. at 1550 nm and for a Fabry-Perot laser is generally about0.5 nm/° C. The wavelength of the light source at a particulartemperature can be expressed as follows: λ_(T)−(TT−T_(LS))(Δλ_(LS)).Other suitable light sources have a rate of modulation wavelength shiftgreater than 0.05, 0.1, or 0.2 nm/° C. and/or less than 0.3, 0.5, or 0.7nm ° C.

Variables in the fabrication process generally produce modulators havinga range of modulation wavelengths at a particular temperature. Forinstance, a batch of modulators will generally have modulationwavelengths that are equal to the desired modulation wavelengths+/−amanufacturing tolerance. The manufacturing tolerance can be indicated bya multiple of the standard deviation. For instance, a Franz Keldyshmodulator constructed according to FIG. 2E through FIG. 2G generally hasa manufacturing tolerance (MT) of about 7.5 nm where 7.5 nm representsthree times the standard deviation. The presence of this manufacturingtolerance reduces the amount that the wavelength of a light signal beingreceived by the modulator can shift while still reliably falling withinthe operating bandwidth (OBW) for each of the modulators. For instance,a light signal that shifts by less than a permissible range (PR) willstill reliably have a wavelength that falls within the operatingbandwidth (OBW) of a modulator fabricated with the above manufacturingtolerance and can accordingly be efficiently modulated by the modulator.The permissible range (PR) can be determined as ((OBW−2MT)/2).

The difference between the modulation wavelength and the channelwavelength must be less than or equal to the permissible range (PR) ofthe modulator in order for the modulator to reliably provide efficientmodulation of the light signal. Accordingly, under these conditions, itcan be stated that[λ_(T)−(TT−T_(m))(Δλ_(m))]−[λ_(T)−(TT−T_(LS))(Δλ_(LS))]≦PR or(TT−T_(LS))(Δλ_(LS))−(TT−T_(m))(Δλ_(m))≦PR. Solving for T_(LS) providesthat T_(LS)≧TT−[PR−(TT−T_(m))(Δλ_(m))]/(Δλ_(LS)). When the electronicshold the temperature of the modulator constant at T_(th), thisexpression becomes T_(LS)≧TT−[PR−(TT−T_(th))(Δλ_(th))]/(Δλ_(LS)). Ininstances where the threshold temperature is equal to the designtemperature (TT), this expression reduces to T_(LS)≧TT−[PR/(Δλ_(LS))] orT_(LS)≧TT−[(OBW/2−MT)/(Δλ_(LS))]. Using the above numbers for a DFBlaser where the threshold temperature is equal to a design temperatureof 60° C. shows that the light source temperature (T_(LS)) can fall aslow as [60° C.−[(35 nm/2−7.5 nm)]/(0.08 nm/° C.)]=−65° C. before thechannel wavelength falls outside of the permissible range (PR) of themodulator. Accordingly, efficient modulation of the light signalproduced by the light source can still be achieved when the light sourcetemperature (T_(LS)) drops to −65° C. However, TL is generally about 0°C. As a result, the threshold temperature can actually be reduced belowthe design temperature. For instance, a threshold temperature of 54° C.permits the light source temperature (T_(LS)) to fall as low as −8° C.before the channel wavelength falls outside of the permissible range(PR) of the modulator. The ability of the threshold temperature to bebelow the design temperature reduces the power requirements associatedwith the heater.

As noted above, the electronics can refrain from operating the heaterwhen the temperature of the modulator would be above the thresholdtemperature without the operation of the heater. Substituting the abovenumbers into (TT−T_(LS))(Δλ_(LS))−(TT−T_(m))(Δλ_(m))≦PR shows that thetemperature of the light source and the modulator can concurrently be ashigh as about 74° C. while still having a wavelengths that fall withinthe permissible range (PR). However, the upper end of the operationalambient temperature range (TH) is generally about 70° C. As a result,the operation of the modulator and light sensor can drive thetemperature of both of these components up by an additional 4° C. whilestill achieving efficient modulation of the light signal. Accordingly,the method of operating the heater provides efficient light signalmodulation across the entire operational ambient temperature range (TH).

Simulation results have shown that for a heater that is 55 μm long usedwith a modulator having a ridge with of 1 μm, a ridge height of 2.7 μm,and a slab region thickness of 0.3 μm, the power requirements for aheater constructed as disclosed above are about 1-2 mW/° C. Accordingly,when the temperature of a modulator would be at 0° C. without operationof the heater, a power in a range of 60 to 120 mW would be needed tokeep the temperature of the modulator at a threshold temperature of 60°C. and a power of only about 54 to 108 mW would be needed to keep thetemperature of the modulator at a threshold temperature of 54° C. Since0° C. is generally the bottom of the operational ambient temperaturerange, the maximum power requirement for the heater is less than 120 mW,108 mW, 80 mW, 60 mW or 54 mW.

Although the device is disclosed as having a single modulator andheater, this is for illustrative purposes and a single device will oftenhave more than one modulator that includes a heater constructed and/oroperated as disclosed above. Examples of a single device that includesmultiple light sources and multiple modulators can be found in U.S.patent application Ser. No. 14/048,685, filed on Oct. 8, 2013, andentitled “Use of Common Active Materials in Optical Components” and inother patent applications that are incorporated into this disclosure.Different heaters on a single device can be operated using the samemethod variables or using different method variables. For instance,different heaters can be operated with different threshold temperaturesor can be operated with the same threshold temperature. Accordingly, thedifferent modulators can be at different temperatures.

Although FIG. 1A and FIG. 1B illustrate a waveguide that connects thelight source directly with a modulator, the device need not include alight source as is disclosed above. Further, the device can beconstructed such that the modulator receives a light signal thatincludes at least a portion of the light generated from one or morelight sources. Accordingly, other components can be optically betweenthe light source and the modulator. For instance, the device can includea multiplexer that multiplexes light signals from multiple light sourcesinto a second light signal that is received by the modulator constructedas disclosed above. Additionally or alternately, the device can includea demultiplexer that receives a light signal from multiple differentlight sources and demultiplexes the light signal into multiple secondlight sources such that at least one of the second light signals isreceived by the modulator constructed as disclosed above. Accordingly,multiplexers and demultiplexers can be positioned between a light sourceand a modulator that receives at least a portion of the light outputfrom the light sensor. Other examples of components that can beoptically between a light source and a modulator that receives at leasta portion of the light output by the light source include, but are notlimited to, amplifiers, switches, combiners, splitters, y-junctions,optical taps, in-line photodetectors and polarization rotators.

Although the above heater is disclosed as generating heat through theapplication of electrical energy to the heater, other heating mechanismscan be employed. For instance, the heater can guide a heated liquid orcan be a source of a light.

Although the device is disclosed in the context of asilicon-on-insulator platform, the device can be constructed on otherplatforms.

Although the above modulators are disclosed as having a single heater, amodulator can include more than one heater or more than one heatingelement. For instance, a heater can include multiple resistors connectedin series or in parallel. Additionally or alternately, the above devicescan include a single thermal conductor rather than multiple thermalconductors.

Although the one or more thermal conductors, heater, and modulator aredisclosed in the context of ridge waveguides, the disclosed thermalconductor, heater, and modulator constructions can be employed withother waveguide types. For instance, the one or more thermal conductors,heater, and/or modulator can be constructed using the ridge associatedwith waveguides such as channel waveguides and buried channelwaveguides.

Although the heater is disclosed as being positioned on the ridge of amodulator, the heater can be positioned on the ridge of other activeoptical components such as light sensors and light sources such as aredisclosed in U.S. patent application Ser. No. 13/506,629. Accordingly,the conductors 43 and heaters disclosed above can also be employed withactive optical components such as light sensors and light sources suchas are disclosed in U.S. patent application Ser. No. 13/506,629. Forinstance, the light sensors and light sources disclosed in U.S. patentapplication Ser. No. 13/506,629 can be substituted for the modulatorsdisclosed above.

Other embodiments, combinations and modifications of this invention willoccur readily to those of ordinary skill in the art in view of theseteachings. Therefore, this invention is to be limited only by thefollowing claims, which include all such embodiments and modificationswhen viewed in conjunction with the above specification and accompanyingdrawings.

1. An optical device, comprising: a optical modulator positioned on abase, the modulator including a ridge extending upward from the base andthe ridge including an electro-absorption medium through which lightsignals are guided, and a thermal conductor positioned so as to conductthermal energy away from the ridge, a distance between the thermalconductor and the ridge changing along at least a portion of the ridge.2. The device of claim 1, wherein a shortest distance between thethermal conductor and the ridge is greater than 0 and less than 1.5micron.
 3. The device of claim 2, wherein the longest distance betweenthe thermal conductor and the ridge is greater than 0.5 micron.
 4. Thedevice of claim 1, wherein a shortest distance between the thermalconductor and the ridge is greater than or equal to 0 and ratio of thelongest distance:shortest distance is greater than 1.5:1.
 5. The deviceof claim 1, wherein the thermal conductor covers more than 4,000 micronsof the device.
 6. The device of claim 5, wherein at least a portion ofthe thermal conductor has a thickness greater than 0.1 micron.
 7. Thedevice of claim 1, wherein the thermal conductor has a thermalconductivity greater than 200 W/(m K).
 8. The device of claim 1, whereinthe modulator has an entry side through which light signals enter themodulator and an exit side through which light signals exit themodulator, the distance between the thermal conductor and the ridgeincreases moving from the entry side toward the exit side for at least aportion of the ridge.
 9. The device of claim 1, wherein the modulatorhas an entry side through which light signals enter the modulator, and ashortest distance between the thermal conductor and the ridge is locatedat the entry side of the modulator.
 10. The device of claim 9, whereinthe modulator includes doped regions that conduct electrical currentduring operation of the modulator and the doped regions define the entryside of the modulator.
 11. The device of claim 9, wherein the thermalconductor extends past the entry side of the modulator.
 12. The deviceof claim 1, wherein the thermal conductor carries an electrical currentduring operation of the modulator.
 13. The device of claim 1, alight-transmitting medium positioned on the base, the electro-absorptionmedium positioned to receive light signals through an input side of theelectro-absorption medium, and the thermal conductor extending acrossthe input side of the electro-absorption medium.
 14. The device of claim1, further comprising: a heater positioned on the ridge.
 15. The deviceof claim 14, wherein the modulator has an entry side through which lightsignals enter the modulator, and the heater extends across the entryside of the modulator.
 16. The device of claim 14, wherein theelectro-absorption medium is positioned to receive light signals throughan input side of the electro-absorption medium, and the heater extendsacross the entry side of the electro-absorption medium.
 17. The deviceof claim 1, wherein the ridge is a ridge of the electro-absorptionmedium.
 18. The device of claim 17, wherein the ridge ofelectro-absorption medium is positioned between slab regions of theelectro-absorption medium and includes lateral sides that extend upwardsfrom the slab regions of the electro-absorption medium, and at least aportion of the thermal conductor is positioned over one of the slabregions such that the slab region is between the thermal conductor andthe base.
 19. The device of claim 1, wherein the thermal conductor isone of multiple thermal conductors and the ridge is located between thethermal conductors.